After a well has been drilled and is an open hole but prior to casing the hole, one formation evaluation procedure involves taking core samples from the sidewall of the well. A coring gun assembly uses an explosive to fire a projectile into the sidewall to cut a cylindrical divot or core from the formation. The coring gun assembly is then retrieved to the surface with one or more cores captured in one or more projectiles, sometimes called barrels or bullets, and the cores are thus analyzed to determine whether or not hydrocarbons are present in them. Thus, the coring gun assembly is an elongate structure which fits in the open hole and which supports one or more bullets. Each bullet is received in a firing chamber and is supported for firing by means of an explosive mechanism behind the bullet. The bullet is supported on a slack retrieval cable so that the bullet is not lost. As wells become deeper and the explosives are exposed to hotter fluids in the well, the operating pressures become higher and the operating temperatures become more severe. Moreover, such explosives must be used in a sealed atmosphere to enable operation without wetting either the igniter charge or the main explosive charge. As these circumstances become more severe, it is more difficult to provide a rugged structure which is able to tolerate pressures as high as 20,000 psi and temperatures ranging up to 500.degree. F. and yet provide a relatively simple mechanism which provides for safe firing. The present structure is such a device. As will be discussed in greater detail, the present structure provides an electrically fired mechanism. There is a protruding metal pin which is enclosed in a glass or ceramic sleeve fused to it to insulate the metal pin. The metal pin is received in a surrounding shell which enables all of the structure to be positioned in the coring gun at a firing chamber. More specifically, the metal pin connects with a bridge element disk of circular construction. It is positioned immediately adjacent to an electrically ignited explosive charge and that in turn is positioned adjacent to another explosive charge. The larger and last charge serves as the explosive propellant which fires the bullet from the chamber. It is especially important to provide this type of structure which operates in the rugged environment previously described because the structure is able to operate at high ambient pressures and temperatures without risk of accidental discharge or mechanical failure. Moreover, it is structurally simple and is able to be assembled with the coring gun assembly, the individual bullet, and the retrieval cables. More will be noted regarding this on a detailed discussion of the preferred embodiment which is set forth below.